In the field of data storage, there are two general types of storage devices. The first type of storage device is volatile memory. Volatile memory loses stored information the instant power is removed from the volatile memory circuit. The second type of storage device is non-volatile memory. Non-volatile memory retains stored information even after power is removed from the non-volatile memory circuit. Some non-volatile memory designs permit reprogramming, while other designs only permit one-time programming. Thus, one form of non-volatile memory is a One-Time Programmable (OTP) memory.
An OTP memory may contain an antifuse. An antifuse is a device that continuously conducts after having been once subjected to a voltage in excess of a threshold voltage, also known as a programming voltage. In other words, an antifuse conducts only after it has been “blown.” An antifuse may be formed from a conventional metal-oxide-semiconductor field-effect transistor (MOSFET).
There are several problems with a conventional OTP memory. The conventional OTP memory is expensive and has field failures due to unnecessary manufacturing defects caused by circuit complexity. Also, the conventional OTP memory is easily reprogrammed while being reverse engineered with a focused ion beam (FIB). The conventional OTP memory is not easily reduced from one process feature size to a smaller process feature size. Further, conventional read/write circuitry does not allow random access to an array of antifuses to determine status of a specific antifuse prior to programming of that antifuse.
An OTP memory should be as small as possible to maximize memory density. However, conventional OTP memory circuits are large, and thus do not maximize memory density. Further, the large size of conventional OTP memory circuits also causes them to be expensive. In addition, conventional OTP memory circuits tend to return unreliable validity test results because circuits that verify and determine an operating margin of the antifuse are separate from traditional read/write circuitry of the memory cell.
Thus, what is needed is an OTP memory that is capable of being randomly accessed prior to programming, is FIB attack-resistant, is less expensive, and that overcomes the other shortcomings described above.
The embodiments are described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.